Unless you plan on tweeting from the great beyond, you might want to consider who gets custody of your Facebook and Twitter accounts before you die.
While not always apparent to my colleagues, I like to plan ahead. I drafted my first will when I was eleven. There have been many revisions since then with the most recent being prompted by the excellent New York Times article about what happens to your Facebook account when you die. The library has many resources available to help with will and estate planning including the suite of products by Nolo Press.